Borehole measurement devices may be used to determine formation characteristics surrounding the borehole and are typically used in wellbores drilled for the purpose of extracting natural resources, such as hydrocarbon, from the formations surrounding the borehole. Borehole measurement devices may use different types of measurements, for example, a borehole measurement device may use gamma measurements, thermal neutron measurements, resistivity measurements or other types of measurements.
FIG. 1 depicts the use of a borehole measurement device in determining formation characteristics of an existing vertical borehole. As depicted, a borehole 102 is drilled through a plurality of formations 104a, 104b, 104c, 104d, 104e. A measurement device 106 may be lowered down the borehole to determine one or more characteristics. The measurement device 106 may include, for example, a neutron source 108 and one or more neutron detectors 110a, 110b, such as gamma detectors, thermal neutron detectors, or epithermal neutron detectors. The measurement characteristics may be associated with the height of the measurement within the borehole. Measurements in vertical boreholes may be made after the borehole is drilled or as it is being drilled.
Boreholes may also include a horizontally drilled portion. For horizontal boreholes, the direction of drilling can be controlled to ‘steer’ the borehole. The direction of the drilling may be controlled using geo-navigation. Geo-navigation may utilize rock cutting analysis and/or resistivity and/or natural gamma ray measurements to determine formation characteristics of the surrounding formations and so determine an appropriate or desirable drilling direction. Rock cutting analysis generally involves a well site geologist performing an analysis on the rock cuttings during drilling. The drilling direction is then based on the results of the analysis. When utilizing down hole measurements, such as resistivity and natural gamma ray, the measurement analysis can be used to make decisions on well path or well architecture.
In geo-navigation, it is desirable to be at the front of the drilling string, or as close to the front of the drilling string as possible, and to measure formation characteristics as far ahead of the drill bit as possible. In order to provide directional information, it is necessary to be able to provide measurements in the radial direction of the drilling string. Typically, the directional measurements have not involved nuclear-based measurements due to the omni-directional nature of the measurements. Gamma detectors can provide radial measurements through the use of shielding material having a window in one location. While a windowed gamma detector can provide radial measurements, it would be desirable to use neutron measurements such as thermal or epithermal measurements as these neutrons are able to penetrate further into formations. However, the thickness of shielding required to shield neutrons is large and as a result may be impractical to use in a down hole application.
It is desirable to make neutron measurements in a radial direction.